This invention pertains to nanoscale conductive devices and, in particular, to metallized nanostructures particularly useful as electron field emitters and nanoscale conductors.
Field emitting devices are useful in a wide variety of applications. A typical field emitting device comprises a field emitting assembly composed of a cathode and a plurality of field emitter tips. The device also typically includes a grid spaced relatively closely to the emitter tips and an anode spaced relatively farther from the tips. Voltage induces emission of electrons from the tips, through the grid, toward the anode. Applications include microwave tube devices, flat panel displays, klystrons and traveling wave tubes, ion guns, electron beam lithography, high energy accelerators, free electron lasers, and electron microscopes and microprobes. See, for example, Semiconductor International, December 1991, p.46; C. A. Spindt et al., IEEE Transactions on Electron Devices, vol. 38, pp. 2355 (1991); I. Brodie and C. A. Spindt, Advances in Electronics and Electron Physics, edited by P. W. Hawkes, vol. 83, pp. 1 (1992); and J. A. Costellano, Handbook of Display Tehnology, Academic Press, New York, pp. 254 (1992), all of which are incorporated herein by reference.
A conventional field emission flat panel display comprises a flat vacuum cell having a matrix array of microscopic field emitters formed on a cathode and a phosphor coated anode disposed on a transparent front plate. An open grid (or gate) is disposed between cathode and anode. The cathodes and gates are typically intersecting strips (usually perpendicular) whose intersections define pixels for the display. A given pixel is activated by applying voltage between the cathode conductor strip and the gate conductor. A more positive voltage is applied to the anode in order to impart a relatively high energy (400-5,000 eV) to the emitted electrons. For additional details see, for example, the U.S. Pat. Nos. 4,940,916; 5,129,850; 5,138,237 and 5,283,500, each of which is incorporated herein by reference.
A variety of characteristics are advantageous for field emitting assemblies. The emission current is advantageously voltage controllable, with driver voltages in a range obtainable from xe2x80x9coff the shelfxe2x80x9d integrated circuits. For typical CMOS circuitry and typical display device dimensions (e.g. 1 xcexcm gate-to-cathode spacing), a cathode that emits at fields of 25 V/xcexcm or less is generally desirable. The emitting current density is advantageously in the range of 1-10 mA/cm2 for flat panel display applications and  greater than 100 mA/cm2 for microwave power amplifier applications. The emission characteristics are advantageously reproducible from one source to another and advantageously stable over a long period of time (tens of thousands of hours). The emission fluctuations (noise) are advantageously small enough to avoid limiting device performance. The cathode should be resistant to unwanted occurrences in the vacuum environment, such as ion bombardment, chemical reaction with residual gases, temperature extremes, and arcing. Finally, the cathode manufacturing is advantageously inexpensive, e.g. devoid of highly critical processes and adaptable to a wide variety of applications.
Previous cathode materials are typically metal (such as Mo) or semiconductor (such as Si) with sharp tips. While useful emission characteristics have been demonstrated for these materials, the control voltage required for emission is relatively high (around 100 V) because of their high work functions. The high control voltage increases damage due to ion bombardment and surface diffusion on the emitter tips and necessitates high power densities to produce the required emission current density. The fabrication of uniform sharp tips is difficult, tedious and expensive, especially over a large area. In addition, these materials are vulnerable to deterioration in a real device operating environment involving ion bombardment, chemically active species and temperature extremes.
Diamond emitters and related emission devices are disclosed, for example, in U.S. Pat. Nos. 5,129,850, 5,138,237, 5,616,368, 5,623,180, 5,637,950 and 5,648,699 and in Okano et al., Appl. Phys. Lett. vol. 64, p. 2742 (1994), Kumar et al., Solid State Technol. vol. 38, p. 71 (1995), and Geis et al., J. Vac. Sci. Technol. vol. B14, p. 2060 (1996), all of which are incorporated herein by reference. While diamond field emitters have negative or low electron affinity, the technology has been hindered by emission non-uniformity, vulnerability to surface contamination, and a tendency toward graphitization at high emission currents ( greater than 30 mA/cm2).
Nanoscale conductors (xe2x80x9cnanoconductorsxe2x80x9d) have recently emerged as potentially useful electron field emitters. Nanoconductors are tiny conductive nanotubes (hollow) or nanowires (solid) with a very small size scale of the order of 1.0-100 nm in diameter and 0.5-100 xcexcm in length. Nanoconductors typically have high aspect ratios ( greater than 1,000) and small tip radii of curvature (1-50 nm). These geometric characteristics, coupled with the high mechanical strength and chemical stability, make nanoconductors attractive electron field emitters. Carbon nanotube emitters are disclosed, for example, by T. Keesmann in German Patent No. 4,405,768, and in Rinzler et al., Science, vol. 269, p.1550 (1995), De Heer et al., Science, vol. 270, p. 1179 (1995), Saito et al., Jpn. J. Appl. Phys. Vol. 37, p. L346 (1998), Wang et al., Appl. Phys. Lett., vol. 70, p. 3308, (1997), Saito et al., Jpn. J. Appl. Phys. Vol. 36, p. L1340 (1997), Wang et al., Appl. Phys. Lett. vol. 72, p 2912 (1998), and Bonard et al., Appl. Phys. Lett., vol. 73, p. 918 (1998), all of which are incorporated herein by reference. Other types of nanoconductors such as Si or Ge semiconductor nanowires can also be useful because of the electrical field concentrating, high-aspect-ratio geometry, although they tend to have an outer surface of insulating oxide. See Morales et al. Science, Vol. 279, p. 208, (1998). Nonconductive materials form similar nanotubes and nanowires with similar favorable geometric features but are not presently used for emission or conduction. The term xe2x80x9cnanostructuresxe2x80x9d will be used herein to encompass nanotubes and nanowires whether conductive or nonconductive.
Nanostructures are typically grown in the form of randomly oriented, needle-like or spaghetti-like powders that are not easily or conveniently incorporated into field emitter devices. Due to this random configuration, the electron emission properties are not fully utilized or optimized. Ways to grow nanostructures in an oriented fashion on a substrate are disclosed in Ren et al., Science, Vol. 282, p. 1105 and Fan et al., Science, Vol. 283, p. 512, both of which are incorporated herein by reference.
In the design and fabrication of efficient and reliable conductors and electron field emitters, a stable electrical continuity from the source of electrical power to the electron emitting tips is important. A combination of two conditions, i.e. that the nanostructures conduct along their entire length and that the substrate surface exhibit continuous conductivity, should be met to ensure a continuous transport of electrons to the field-emitting tips. (A third condition of electrical continuity from underneath the substrate to the top surface of the substrate should also be met if the electrical contact from the power supply is to be made on the bottom surface of the substrate). However, there are occasions when one or both conditions are not met.
Carbon nanotubes are the preferred nanostructures for electron field emission. FIGS. 1(a)-1(c) are schematic molecular models showing various known configurations of carbon nanotubes. Single-wall nanotubes can be metallic with the xe2x80x9carmchairxe2x80x9d configuration of the carbon atoms C (FIG. 1(c)). See M. S. Dresselhous et al., Science of Fullerines and Carbon Nanotubes, Chapter 19, p. 758 and p. 805-809, Acdemic Press, San Diego, 1996. It is also known that the nanotube atomic arrangements and hence the electrical properties sometimes vary drastically along the length of a single tubule. See P. G. Collins, et al., Science, Vol. 278, p. 100 (Oct. 3, 1997). Thus efficient electron transport from the cathode substrate to the emitting tip is not always guaranteed.
Furthermore, nanostructures can be comprised of nonconductive material or the substrate itself can be electrically insulating, e.g. a porous ceramic substrate. See Li et al., Science, Vol. 274, p. 1701 (Dec. 6, 1996). In such a case, there will be a discontinuity in electron transport.
Accordingly there is a need for a nanostructure assembly that provides continuous electron transport from the cathode power supply to the field-emitting tips and a need for a method of making such an assembly.
In accordance with the invention, an improved conductive nanostructure assembly comprises an array of metallized nanostructures disposed on a conductive substrate. The substrate can also be metallized. Such assemblies provide continuous electron transport from the substrate to the tips of the nanostructures. Several ways of making such assemblies are described along with several devices employing the assemblies.